This invention relates to electrophotographic development utilizing magnetic toner particles which are applied from magnetic brush to the electrostatic latent image without use of a carrier material.
There have been suggested systems for magnetic development not utilizing carrier material. One such system was described in U.S. Pat. No. 2,846,333 to Wilson. Wilson et al disclosed the use of magnetic brush to apply toner particles formed of ferrites and resin material to develop electrostatic latent images. The difficulty with this process was the conductivity of the toner made electrostatic transfer difficult.
A further development of magnetic development without carriers is illustrated by Kotz, U.S. Pat. No. 3,909,258 wherein an electrostatic development process utilizing a magnetic brush without carrier is illustrated. A toner suitable for use in the Kotz process is disclosed in U.S. Pat. No. 3,639,245 to Nelson wherein a dry toner powder having specific electric conductivity is disclosed. The toner of Nelson is formed by blending magnetite with the resin and then after blending pulverizing to a small particle size. The particles are then mixed with conductive carbon black which is imbedded in the surface of the particle and then a small particle size SiO.sub.2 agent is mixed into the toner to improve the flowability. The toner of Nelson suffers the disadvantages that it does not transfer well from a photoconductive substrate to plain bond paper.
Therefore there is a continuing need for magnetic toners suitable for use in one component magnetic development systems. There is a need for toners suitable for high speed development that also have good electrostatic transfer characteristics for transfer from a photoconductive surface to plain bond paper. There is further a need for a magnetic pigment coating which aids field dependence without introducing undesirable properties such as humidity sensitivity into the toner. There is further a need for methods of rendering pigments hydrophobic and more easily dispersable in polymers.
There is a need for toners which after they are transferred to paper adhere sufficiently well by electrostatic forces that image disturbances (blur) do not readily occur on handling the unfused toner image prior to fixing.
In copending application, Ser. No. 792,636, filed May 2, 1977, titled: "Single Component Magnetic Toner" which is hereby incorporated by reference, it is disclosed that coating of magnetic pigments with fatty acids prior to mixing with resin and spray drying to form a toner will produce field dependence toner. However, there is a continuing need for additional field dependent toners.
There is also a need for coated magnetic pigments in the printing industry. There is a continuing need for pigment having hydrophobic characteristics such that it is suitable for use in multilith inks.